`on.exit` in tests?

[brodieG]

Currently not entirely sure when on.exit would get called. Maybe after eval of that same expression? Based on this it would seem so:

> eval(quote(on.exit(cat('hello\n'))))
hello

If so do we need another mechanism to register on.exit? Maybe not; after all typically on.exit is designed for when something fails, and those failures should be very rare with unitizer. Notwithstanding, it may still be desirable to have that fallback for stuff like Sys.setlocale, etc.